The present invention relates to the process for producing silica by decomposition of an organosilane.
More particularly, the present invention relates to a process for producing an optical preform of silica by decomposition of an organosilicon compound of formula (I), as indicated later.
Many processes for producing metal oxides by decomposition of suitable reagents in the vapour state are known in the prior art. In general, these processes require a feed solution containing the metal compound whose oxide it is desired to obtain, means for evaporating the said solution, means for conveying the vapours obtained, oxidizing means or combustion means. A mixture of finely divided spherical aggregates referred to as xe2x80x9csootxe2x80x9d is thus obtained, which can be collected in various ways.
Among the said processes for producing metal oxides, the ones of particular interest are those relating to the production of high-purity silicon dioxide, or silica (SiO2). This high-purity requirement is particularly essential when the silica is used in highly sophisticated sectors such as, for example, the production of semiconductors and optical fibres. The reason for this is that it is well known that, in order for an optical fibre to be able to ensure high-quality transmission of the optical signals, i.e. a low level of attenuation, the silica of which it is composed must have a very high degree of purity.
According to one method for producing optical fibres, the soot is deposited onto a horizontally rotating bar, while the burner, and thus the flame, translates cyclically (outside vapour deposition; OVD) or else on a bar which rotates and moves vertically, while the burner, and thus the flame, remains fixed at the lower end of the bar (vapour axial deposition; VAD). Typically, the said bar (mandrel) is cylindrical and is made of high-purity glass. After the desired amount of soot has been deposited, the central bar is removed and roughcast thus obtained is heated, dehydrated and solidified. The component thus obtained is known as a xe2x80x9cpreformxe2x80x9d, and an optical fibre is then drawn therefrom by means of a suitable device which works under controlled conditions of melting point, tension, speed and diameter of the fibre.
The industrial method used for many years to produce high-purity silica is based on the decomposition of silicon tetrachloride (SiCl4), but this decomposition has the drawback of entailing the formation of toxic and corrosive gaseous by-products such as chlorine (Cl2) and hydrochloric acid (HCl). A plant for producing silica by this method must therefore be adequately fitted with devices for cutting down the said toxic gases, and requires constant maintenance with substantial increase of the costs.
These drawbacks have directed research towards halogen-free materials.
U.S. Pat. No. 5,043,002 gives a review of various halogen-free organosilicon compounds. According to that document, among all halogen-free organosilicon compounds, those which are most suitable for producing high-purity silica are the siloxanes and, among all the siloxanes investigated, the most suitable proved to be octamethylcyclotetrasiloxane (xe2x80x94[SiO(CH3)2]4xe2x80x94).
Experiments carried out by the inventors of the present invention have found, however, that the high boiling point (175xc2x0 C.) of octamethylcyclotetrasiloxane (OMCTS) causes serious drawbacks such as, for example:
a) the need to heat the feed conduits to a temperature greater than 175xc2x0 C. in order to avoid local condensations of OMCTS. This requirement creates considerable difficulties since no apparatus currently exists which is capable of measuring the flow rate of a vapour at a temperature above 130-140xc2x0 C. Thus, it is necessary to measure the flow rate of liquid OMCTS upstream of the bubbling device. However, measuring the compound in the liquid phase does not allow an accurate control of the actual amount of vapour subsequently formed in the bubbling device per unit of time and this does not allow stabilization of the system by means of controlling the flow rates;
b) the need for specific evaporation systems such as, for example, a flash vaporizer of the type described in U.S. Pat. No. 5,078,092 or the film evaporator described in U.S. Pat. No. 5,707,415;
c) the thermal decomposition of the OMCTS, while passing through the feed conduits, into non-volatile polymer residues which are deposited in the conduits and thus block them. Besides generating the said residues, the said thermal decomposition also has an adverse effect on the optical quality of the preform obtained.
These and other drawbacks associated with the use of polyalkylsiloxanes in the vapour state are also described in patent application WO 97/22553, which reports that, during the feeding of polyalkylsiloxanes, in the vapour state, into the burner, high-molecular-weight species are deposited, in the form of gel, in the line which conveys the reagents into the burner or in the burner itself. This leads to a reduction in the speed of deposition of the xe2x80x9csootxe2x80x9d preform and also to imperfections which produce defective or unusable optical fibres (from page 7, line 33 to page 8, line 7). In another passage of that document, it is pointed out that the abovementioned formation of deposits in gel form is due to high-boiling impurities present in the polyalkylsiloxanes and that the formation of these deposits hinders the control and reproducibility of the process. This problem is all the more serious when an oxidant carrier gas, such as oxygen, is present in the flow of the polyalkylsiloxane vapour, since the oxidants can catalyse the polymerization of the polyalkylsiloxane (from page 9, line 26 to page 10, line 18).
It has now been found, surprisingly, that an organosilicon compound of formula (I), as shown later, comprising at least two silicon atoms and containing no oxygen atoms, does not have, in the vapour state, the abovementioned drawbacks. To be specific, this gives a silica of high purity with high yields of deposition and requires very simple, inexpensive and easy-to-maintain apparatus.
This is all the more surprising when one considers that the use of silane was not recommended on account of the violence of its combustion reaction (U.S. Pat. No. 5,043,002, column 2, lines 25-36). The Applicant has moreover observed that a silane compound free of oxygen but containing a single silicon atom (in particular tetramethylsilane) has a series of drawbacks associated with the difficulties in managing and controlling the combustion reaction, which make it complicated to use as a reagent for producing silica. It has moreover been observed that this compound, under normal process conditions, forms silica particles of small diameter, while the silica deposited is of low density. These two combined phenomena give rise to a vitreous mass which is too fragile for the subsequent treatments required to obtain an optical preform.
A first aspect of the present invention thus consists of a process for producing a high-purity optical silica preform, comprising
a) vaporization of an organosilicon compound;
b) thermal decomposition of the said organosilicon compound in the vapour state, to give amorphous fused silica particles;
c) deposition of the said amorphous fused silica particles on a support; and characterized in that the said organosilicon compound has the following formula 
in which
R1, R2 and R3 are each, independently, hydrogen, methyl, ethyl, propyl, isopropyl or a group of formula xe2x80x94Sixe2x80x94(R5R6 R7), where R5, R6 and R7 are each, independently, methyl, ethyl, propyl or isopropyl, and
R4 is a group of formula xe2x80x94(CH2)mxe2x80x94Sixe2x80x94(R5R6 R7), where R5, R6 and R7 are as defined above and m is an integer between 0 and 3.
Preferably, at least two of the groups R1, R2 and R3 are other than hydrogen. Among these compounds, the ones which are preferred are those which are liquid at room temperature, those with a boiling point of less than about 140xc2x0 C., preferably between about 70xc2x0 C. and about 140xc2x0 C., being particularly preferred.
Examples of compounds of formula (I) are:
(CH3)3xe2x80x94Sixe2x80x94Sixe2x80x94(CH3)3 (hexamethyidisilane)
[(CH3)3xe2x80x94Si]3xe2x80x94Sixe2x80x94H (tris(trimethylsilyl)silane)
(CH3)3xe2x80x94Sixe2x80x94CH2xe2x80x94Sixe2x80x94(CH3)3 (bis(trimethylsilyl)methane)
Preferably, the compound of formula (I) is hexamethyldisilane.